1. Field of the Invention
The present invention relates to a method for reducing the risk of a call connection failure, and a system to perform the method.
2. Discussion of the Background
A variety of wireless communication services using a wireless network are available due to the development of computer and communication technologies. A basic wireless communication service may be a wireless voice call service that permits mobile terminal users to make a wireless voice call anytime and anywhere. Accordingly, a mobile communication service subscriber may speak to another user anytime and anywhere, including while moving, using a wireless voice call service.
H.324 is an International Telecommunication Union (ITU) protocol standard for multimedia communication over generalized switched telephone networks (GSTN). H.324M is an extension of H.324 for operations over mobile networks, and 3G-324M is a recommendation by the third generation partnership program (3GPP) defining an adaptation of H.324M for use within 3GPP and also adopted by 3GPP2. “H.324-like” equipment includes devices and systems employing protocols based on or derived from H.324. H.324-like equipment can connect to other H.324-like equipment via switching centers and to non-H.324-like equipment through multimedia gateways.
When a call is made between equipment using H.324, H.324M or 3G-324M, the first stage of the call is to establish an end-to-end bearer between the equipment. This stage is called ‘Call Signaling’ and is outside the scope of H.324, except where modems and the GSTN are used. The second stage of the call is to establish the H.324 session, to provide a way of transporting video, audio, and/or data between the equipment in a format that is known to and supported by the equipment. In order to accomplish this, H.324M makes use of two ITU-T Recommendations.
The first Recommendation is H.223 “Multiplexing protocol for low bit rate multimedia communication”. H.223 specifies a frame-oriented multiplexing protocol that allows the transfer of any combination of digital voice, video, and data (e.g. command and control) information over a single communication link. The H.223 may have a number of modes of operation, specified in Annexes A, B and C of the H.223 Recommendation, that are intended to provide increased resilience in the presence of errors. These modes of operation are also known as Mobile Levels 1, 2, and 3. H.223 without the application of any of these Annexes is also sometimes referred to as operating at Mobile Level 0 (base-line). H.324 includes the concept of Logical Channels, which is a way of providing virtual channels over the circuit switched link. The role of the multiplexer is to combine (multiplex) parts of the data chunks written on the logical channels into frames known as a Multiplexer Protocol Data Unit (MUX-PDU). Logical Channel 0 is used for Command and Control. Data (voice, video, command and control and other general data) is passed to/from the H.223 multiplexer through bitstream chunks called service data units (SDUs). Before being multiplexed, these different SDUs go through Adaptation Layers where extra information may be added for purposes such as error detection, sequence numbering, and/or retransmission requests.
The second Recommendation is H.245 “Control protocol for multimedia communication,” which specifies the syntax and semantics of terminal information messages as well as procedures to use the messages for in-band negotiation at the start of or during communication. The messages cover receiving and transmitting capabilities and preferences, logical channel signaling and control and indication. The messages that are specified in H.245 are expressed in the ITU-T Abstract Syntax Notation (ASN.1) and can be classified as request, response, command, or indication type. H.245 messages are encoded according to the ASN.1 standard before being transmitted. If a sending terminal sends an H.245 message of type “request” to a target terminal, the target terminal should send an appropriate message of type “response” in response. If the response (sometimes referred to as an Ack for Acknowledgement) is not received within a certain time, the sending terminal may re-transmit the request, or may take other appropriate action if no response has been received for repeated requests. Re-transmission of requests may occur a number of times. Many of the H.245 messages associated with call set up are of the “request” type.
H.245 also includes a reliable link layer for proper operation. The principal way of providing the reliable link layer, specified in annex A of H.324, is to use the Simple Retransmission Protocol (SRP) or the Numbered Simple Retransmission Protocol (NSRP), in which one or more H.245 messages, known collectively as a MultimediaSystemControl PDU and in the present document as an H.245 PDU, are formed into SRP Command Frames prior to sending, and the receiving terminal sends an SRP response frame (sometimes referred to as an SRP Ack) to acknowledge correct receipt of each SRP command frame. No further H.245 messages are usually sent by a terminal until the SRP Ack for the last message has been received.
Because an H.245 Response message is sent for each H.245 request message received, and because an SRP Ack is received for every SRP command frame sent, a single H.245 request message may take some time to be conveyed successfully. The H.324 standard allows for multiple H.245 messages to be concatenated into a single SRP command frame. However, this capability is often not implemented, in which case a receiving or called terminal may respond only to the first H.245 message encountered in an SRP command frame. In some cases, terminals that do not support this capability may malfunction upon receiving an SDU containing multiple H.245 requests or responses.
The steps involved in setting up and connecting a typical H.324 call include:
1. Call signaling (bearer establishment). This may be achieved by a modem connection in the case of GSTN, through ISDN, or signaling through mobile switching centers in the mobile case.
2. Mobile level detection (MLD). In this step, a common Mobile Level is agreed on between equipments. This step is performed by H.324 equipment that supports mobile extensions such as H.324M and 3G-324M equipment.
3. Terminal Capability Exchange (TCS)
4. Master Slave determination (MSD)
5. Open/Close Logical Channels (OLC)
6. Multiplexer Table Entries Exchange (MTE)
Steps (3) to (6) are performed using H.245 Messaging, and more specifically, a sequence of H.245 request and response messages as described above.
Further, steps (3) to (6) relate to procedures that are defined by underlying state machines known as Signaling Entities. The relevant signaling entities are:
1. Capability Exchange Signaling Entity (CESE)
2. Master Slave Determination Signaling Entity (MSDSE)
3. Logical Channel Signaling Entity (LCSE)
4. Multiplex Table Signaling Entity (MTSE)
Once these steps have been completed, media (video, audio, and/or data) can flow between the terminals. The H.245 messages flow on the Logical Channel 0, which as previously described is predefined and carried by the multiplexer predefined Multiplex Table Entry 0. Once other Multiplex Table Entries have been exchanged, these can also be used in conjunction with H.245 messages.
The steps above are often handled sequentially. However, this may result in as many as ten H.245 message round trips, including delays, to establish an H.324 session with two logical channels in each direction. In addition, the SRP scheme (or Numbered version—NSRP, if the mobile level is greater than zero) used for H.324/H.245, in which an SRP message is received by the endpoint for every message sent, prior to sending any other message, regardless of whether it is associated with the same Signaling Entity or not, further limits the scope to pipeline messages on the network, and further slows call set up.
For H.324M, the Terminal Capabilities Set request (TCS) step described above is preceded by a mobile level detection/multiplexer synchronization phase. This includes each terminal transmitting a repeating pattern of bits (flags) that indicate the highest mobile level at which the terminal operates. Each terminal examines the flags that it is receiving. If a received flag represent a lower mobile level than the mobile level of the receiving terminal, the terminal drops down to the same lower level. When both terminals are transmitting the same flag sequence this step can be completed.
Arising from the set of procedures described above to establish an H.324M call, when a call is made from a terminal using H.324, a long call set up time may occur. Call set up time is the interval beginning when the call signaling is initiated to the time that the exchange of voice and video commences between an H324-like end-point (H.324, H.324M or 3G-324M) and another terminal, whether H.324-like or not.
The ITU Recommendation H.323 uses H.245 in a similar manner to H.324 for signaling command, control, and indication messages related to a call. Unlike H.324, H.323 is equipped with a number of features to increase the call set up time between H.323 equipment. Similar techniques exist for the IETF Session Initiation Protocol (SIP) protocol.
Thus, there exists a need for techniques to speed up the call set up between H.324 like terminals and other terminals (including servers) either of the H.324 type, or terminals such as H.323 via multimedia gateways that terminates the H.324 side and would have an H.324-like termination in them. The differences between the H.324 protocol (and its extensions such as H.324M and 3G-324M) and H.323 and other protocols mean that additional aspects should be considered when introducing call establishment speed-up techniques for H.324-like terminals. Such differences may include the information about mobile levels where they are used and the messaging and information related to the H.223 multiplexer such as its multiplex table entries, adaptation layers, and so on.
Currently, a ring back tone (RBT) service is the focus of attention. An RBT service enables various types of multimedia data (for example, video and voice) designated by a telephone receiver of a mobile terminal to be replayed in a mobile terminal of a caller or a general telephone. An RBT designated by a telephone receiver is provided to a caller in an RBT service. Voice data, such as new pop songs instead of a traditional mechanical RBT, as well as video data such as a picture or movie, may be used as a multimedia RBT.
If a multimedia RBT service is provided, a telephone receiver may look at or listen to a designated video or voice as opposed to a mechanical RBT, and thus listening and visual satisfaction may increase.
In a conventional video communication, a called terminal assumes that a call is connected and a video call session starts when a telephone receiver answers a phone. Actually, a call channel between an originating subscriber and a call connection video server may be established first, and then switched when a telephone receiver answers a phone.
Accordingly, a calling terminal disconnects an existing video call session and prepares a new session, and in this process a terminating subscriber may receive a message generated at the calling terminal and intended to be transmitted to a call connection video server.
If a called terminal receives such a message, the called terminal may set a sequence number of a Numbered Simple Retransmission Protocol (NSRP) layer of H.324 for handling H.245 message as an initial value. In this instance, the sequence number of the NSRP layer is an erroneously set value, and is a value used by a calling terminal in a session with an existing call connection video server. Accordingly, the called terminal may recognize a sequence number of a normal message, sent after the calling terminal closes a session with the call connection video server, as an error, and thus a video call session may fail due to timeout and the call is not connected.